"two armless, ladder-backed chairs"
Modern ladder-back chair - Credit: Yandle, Flickr
Antique ladder-back chair (date unknown) - Credit: sailko, Wikimedia Commons
A Ladder-backed chair is one whose back consists of two upright posts joined by two or more horizontal slats; thus creating the effect of a ladder.
The style has certainly been in existence since the 17th Century, and continues to be popular today.